Described herein are toners, and developers containing the toners, in particular emulsion aggregation toners, with improved stability of triboelectric charging performance. In particular, the present disclosure is related to methods of grafting metal oxide particles onto polycondensation polymer such as polyester resin. The resin with the covalently linked metal oxide particles can be used as a material for forming an outer surface, or shell, of a toner particle.
Emulsion aggregation (EA) toners are excellent toners to use in forming print and/or xerographic images in that the toners can be made to have uniform sizes and in that the toners are environmentally friendly. Common types of EA toners include polyester based and acrylate based toner.
EA techniques typically involve the formation of an emulsion latex of the resin particles, which particles have a small size of from, for example, about 3 to about 500 nanometers in diameter, by heating the resin, optionally with solvent if needed, in water, or by making a latex in water using emulsion polymerization. A colorant dispersion, for example of a pigment dispersed in water, optionally also with additional resin, is separately formed. The colorant dispersion is added to the emulsion latex mixture, and aggregation is conducted, for example with addition of an aggregating agent or complexing agent, to form aggregated toner particles. The aggregated toner particles are optionally heated to enable coalescence/fusing, thereby achieving aggregated, fused toner particles.
External surface additives are typically added to the surface of the toner particle. Such surface additives include, for example, metal oxides such as silica, which is applied to the toner surface for toner flow, triboelectric enhancement, admix control, improved development and transfer stability and higher toner blocking temperature, and titania, which is applied for improved relative humidity (RH) stability, triboelectric control and improved development and transfer stability.
Toner charging performance may be negatively affected by poor surface additive attachment to a toner particle surface, which can lead to contamination of loose additives as dirt in a machine. Currently, processes to improve surface additive attachment adjust the additive blending conditions in an effort to improve the physical attachment.
Toner charging performance may also be negatively affected by additive impaction, which is observed when toner ages in a development housing, impacting developer flow, and charging, along with cleaning and transfer. Currently, processes to improve additive impaction include redesign of the toner resin for either conventional or EA toners to increase the polymer glass transition temperature (Tg) to produce a tougher, more durable particle surface.